RWB II ROTARY SCREW COMPRESSOR UNITS
INSTALLATION
S70-200 IOM
Page 13
Due to the tendency of the port pressure to fall with de-
creasing compressor capacity, a back-pressure regulator
valve (BPR) is generally required on a flash economizer sys-
tem (Figure 13) in order to maintain some preset pressure
difference between the subcooled liquid in the flash vessel
and the evaporators. If the back-pressure regulator valve is
not used on a flash economizer, it is possible that no pres-
sure difference will exist to drive liquid from the flash vessel
to the evaporators, since the flash vessel pressure will ap-
proach suction pressure at a decreased slide valve position.
In cases where wide swings in pressure are anticipated in
the flash economizer vessel, it may be necessary to add an
outlet pressure regulator to the flash vessel outlet to avoid
overpressurizing the economizer port, which could result in
motor overload. Example: A system feeding liquid to the flash
vessel in batches.
The recommended economizer systems are shown below.
Notice that in all systems there should be a strainer (STR)
and a check valve (VCK) between the economizer vessel
and the economizer port on the compressor. The strainer
prevents dirt from passing into the compressor and the check
valve prevents oil from flowing from the compressor unit to
the economizer vessel during shutdown.
Other than the isolation valve
needed for strainer cleaning, it is
essential that the strainer be the
last device in the economizer line before the compres-
sor. Also, piston-type check valves are recommended
for installation in the economizer line, as opposed to disc-
type check valves. The latter are more prone to gas- pul-
sation-induced failure. The isolation and check valves
and strainer should be located as closely as possible to
the compressor, preferably within a few feet.
Figure 12 - Direct Expansion Economizer System
Figure 11 - Shell and Coil Economizer System
Figure 14 - Multiple Compressor Economizer System
Figure 13 - Flash Economizer System
HIGH
PRESSURE
LIQUID
INTERMEDIATE PRESSURE
GAS TO COMPRESSOR
SUCTION
STR
VCK
SUBCOOLED
HIGH PRESSURE
LIQUID TO
EVAPORATOR
ECONOMIZER
COOLER
ECON1
HV-2
HIGH
PRESSURE
LIQUID
INTERMEDIATE PRESSURE
GAS TO COMPRESSOR
SUCTION
STR
VCK
SUBCOOLED
HIGH PRESSURE
LIQUID TO
EVAPORATOR
ECONOMIZER
COOLER
WIRING
ECON2
HV-2
HIGH
PRESSURE
LIQUID
INTERMEDIATE PRESSURE
GAS TO COMPRESSOR
SUCTION
STR
VCK
CONTROLLED
PRESSURE
SATURATED LIQUID
TO EVAPORATOR
ECONOMIZER
VESSEL
BPR
ECON3
HV-2
INTERMEDIATE PRESSURE
GAS TO COMPRESSOR
SUCTION
STR
VCK
BPR
CONTROLLED PRESSURE
SATURATED LIQUID TO EVAPORATOR
ECONOMIZER
VESSEL
ECON4
HV-2
For refrigeration plants employing multiple compressors on
a common economizing vessel, regardless of economizer
type, each compressor must have a back-pressure regulat-
ing valve in order to balance the economizer load, or gas
flow, between compressors. The problem of balancing load
becomes most important when one or more compressors
run at partial load, exposing the economizer port to suction
pressure. In the case of a flash vessel, there is no need for
the redundancy of a back-pressure regulating valve on the
vessel and each of the multiple compressors. Omit the BPR
valve on the flash economizer vessel and use one on each
compressor, as shown in Figure 14. It is also recommended
that the back-pressure regulating valves, used on economizer
lines, should be specified with electric shutoff option. The
electric shutoff feature is necessary to prevent flow from the
common economizer vessel to the suction side of a stopped
compressor, through the suction check valve bypass line, if
the other compressors and the common economizer vessel
are still operating and the HV2 valve on the suction bypass
is open.
For refrigeration plants using a Packaged Refrigerant Recir-
culation (PRR) unit and a direct expansion (DX) economizer
system it is necessary to operate the liquid feed solenoid on
the PRR unit and the liquid feed solenoid on the DX vessel
off of a common signal to avoid liquid overfeed on the DX
economizer system.
If multiple compressors are operated with a common econo-
mizer vessel, it is necessary to install a back-pressure regu-
lator valve with an electric shutoff option in the vapor line
piped to the compressor's economizer port. If an electric shut-
off is not installed in the economizer vapor line, valve HV-2
must remain closed to avoid a gas bypass from the econo-
mizer line through the suction check valve bypass, back to
the suction line on a compressor that is shut down.
